Bob Nuckolls
AeroElectric Connection
6936 Bainbridge Road
Wichita, KS 67226-1006
Voice/Fax: 316-685-8617
E-mail: nuckolls@aeroelectric.com
Let's Talk About Starter Solenoids
Back in the "good ol' days" starter pinion gear engagement -andelectrical connnection between motor and battery were accomplished
with totally manual mechanisms. My 1941 Pontiac had a starter pedal
on the floor just to the right of the acellerator.
I believe the first starter featuring automatic engagement of the pinion
gear used Bendix drives. If one wished to use a device so modern as a
push button for starting, a contactor was needed to handle high current
switchin g
duties.
Figure 1.
Later,
I
believe it
was GM who tried a scheme to improve on the shady history of Bendix
drives and designed a solenoid engagement system which ALSO
featured an integral contactor for switching motor current. A schematic
for the device is shown in Figure 1. Here's where the "thing that
switches starters" began to be properly called a solenoid. The name
stuck and to this day . In automotive vernacular, even remotely mounted
contactors are sometimes refered to as "solenoids."
The "solenoid" in Figure 1 has two functions: First, the pinion gear
extends to engage the flywheel ring gear. Second: heavy contacts built
into one end of the solenoid assembly close to conduct battery power to
the starter motor. The mechanism was properly called a solenoid
because of its duties as a linear motor with both mechanical and
electrical tasks. Given the extra-ordinary effort required of the solenoid
coil, it might typically draw 8 to 10 amps during engine cranking.
Figure 2.
An interesting aspect of solenoid operation is that it can HOLD a force
with a small fraction of the current required to STROKE the same force
through the length of its travel. Once the armature bottoms-out it's
capable of holding many times the force needed to initially engage the
pinion gear. Designers reasond correctly that the total electrical energy
and size of solenoid required to operated a starter be reduced along with
© 1996, 1997, 1998 Bob Nuckolls, Wichita, Kansas. E-mail: nuckolls@aeroelectric.com. This document may be reproduced mechanically
or electronically for non-profit, educational purposes when produced in its entirety and without modification.
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a small change in design.
In the Figure 2, note that the solenoid coil is wound with TWO coils of
wire. Each winding has the same number of turns . . . one coil is wound
with larger diameter wire. The coil wound with smaller wire is connected
just like the coil in the Figure 1 . . . power from the starter push button is
routed through the winding directly to ground. The second winding also
gets power from the starter push-button . . . but it terminates on the
OUTPUT terminal of the contactor . . . its path to ground is THROUGH
the non-rotating starter motor.
In the Figure 3, I show the starter button closed and electron flow coming
through BOTH solenoid windings in the same direction around the core .
. . they AID each other in exerting a force on the solenoid armature which
begins to move the pinion gear out to engage the ring gear. When the
starter motor is not in motion, it looks pretty much like a dead short to
ground. Current flowing through the large-wire coil is on the order of 25
amps. Current flowing in the small wire coil is lower . . . about 10 amps.
During this time, current through the starter push button is on the order of
35 amps!!! The total magnetic force avaialable to move things is 35*T
ampere-turns.
After the pinion gear engages, the contactor closes and puts a short across
the large-wire coil reducing its current draw to zero. The Figure 4 shows
the contactor closed to (1) apply power from battery to the starter motor
and (2) relieve starter push-button loading by about 25. At this time, the
pinion gear is
engaged ,
Figure 3.
power
is
applide to the
starter motor, and HOLDING current for the solenoid drops to about 10
amps.
When the engine starts, the starter push button is released which
produces and interesting result. Check out what happens in Figure 5.
With no source of power through the starter push button, the
downstream side of the starter contactor becomes the source of power.
The two coils of the starter solenoid are now in series with electron flow
in OPPOSITE directions. Current flow at all points within a series
circuit are the same. Both coils have the same number of turns. Hence,
each coil generates a magnetic field equal to but opposite that of the
other. The net force is zero. The solenoid relaxes shutting off power to
the starter motor and retracting the pinion gear from the ring gear.
Figure 4.
This design has several important operating cosiderations. (1) current
flow through the starter switch for the few milliseconds it takes to close
the starter contactor is quite high . . on the order of 25-35 amps. (2) This
high initialization current produces a snappy, high value engagement
force to move the pinion gear into position and close the starter
contactor. (3) Once the contactor is closed, power to keep the solenoid
energised falls to a small fraction of the initial current flow. (4) when the
start push button is released, the equal but opposite fields produced by
the pair of solenoid windings allows the dis-engagment springs to retract
the pinion gear and shut off the motor with greatest possible dispatch.
© 1996, 1997, 1998 Bob Nuckolls, Wichita, Kansas. E-mail: nuckolls@aeroelectric.com. This document may be reproduced mechanically
or electronically for non-profit, educational purposes when produced in its entirety and without modification.
2
This arrangement has a major disadvantage . . . it's really hard on starter
push buttons. Many of the small import cars that feature this solenoid
system suffer from accelerated wear of the contacts in the key lock
switch. A few years back, one of my wiring diagrams called for starter
switch current to be brought through a 10 amp, essential bus alternate
feed fuse. I tried to eliminate the need for a separate starter contactor by
utilizing the contactor built into the current offering of lightweight
starters. I'd overlooked the 35 amp/50 millisecond pulse required to
engage this type of solenoid . . . the E-Bus alternate feed fuse blew
immediately.
B&C has always recommended installation of a separate starter
contactor and wiring the starter as shown in figure 6. B&C jumpers the
starter solenoid coil terminal to the contactor main terminal on every
starter shipped. Starter contactors offered by B&C and ourselves have
coil resistances of about 4 ohms. Energizing this device from a 12-volt
battery produces starter button currents on the order of 3 amps.
Figure 5.
Figure 6.
© 1996, 1997, 1998 Bob Nuckolls, Wichita, Kansas. E-mail: nuckolls@aeroelectric.com. This document may be reproduced mechanically
or electronically for non-profit, educational purposes when produced in its entirety and without modification.
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